Hi Holly,
I imagine the borderline personality disorder is a big factor here. It’s possible that you may not be in mortal sin due to diminished capacity as a result of your illness, but that is something you’ll need to discuss with a priest.
The Sacraments and prayer is your first line of defense. On our own we cannot avoid sin, which is why we need to pray daily for help. Have you considered the Sacrament of Annointing of the Sick?
In addition to prayer, I would make sure to follow the advice of your doctors and take medication as prescribed. If you are doing that and it is not helping, then let your doctor know. If you’re not already in counseling, you might consider going to discuss ways you can manage the impulse to change religions constantly.
Remember that God loves you very much and this cross of BPD that you bear will work for your good in the long run (see Romans 8:28). When we suffer, we are being perfected.
Also remember that God will not allow you to be tested beyond your strength, and He will give you the grace to overcome temptations (1 Corinthians 10:13).